1997 Pontiac Bonneville bonneville losing pressure

I have a 97 Bonneville 3.8 se and over the past month when I start up the car the rpm would fluctuate then after a couple of mins the rpm would jump up maybe because of asensor which raises about to 3 rpm. After this the rpms usually remain above 1 rpm. After a while of driving ill notice that on straight way roads the car drives perfext but on inclined roads there is a significant drop in pressure and the car barely makes the hill. Also on straight way roads the car has harsh shifting in only first and second gesr. I ran a diagnostic test and got a p1870 code but could this also be due to a bad coil pack